Generally speaking if you fit smaller wheels and tyres to your car you should experience an improvement in ride quality, but it does also depend on the aspect ratio of the tyre - the two digit number in the middle of the tyre size. This represents the height of the sidewall as a percentage of the tyre width, so the higher the number, the taller the sidewall and usually the better the ride comfort.

However, if you switch to 18-inch wheels but fit run flat tyres, some of this benefit will be lost, as run flat tyres have a stiffer sidewall in order to provide the run flat capability.